The invention pertains to a method and system for manufacturing hot-mix asphalt, and in particular to a method and system for manufacturing hot-mix asphalt including recycled asphalt products.
The present invention relates to a transportable hot-mix asphalt manufacturing system and method, and more specifically, to a hot-mix manufacturing system that is relatively compact and transportable to sites where the hot-mix asphalt is to be manufactured. The present invention also relates to compact pollution control systems and pollution control methods for a hot-mix asphalt manufacturing system.
While techniques and systems for manufacturing hot-mix asphalt are generally known, typically the systems used to manufacture the hot-mix asphalt include large, cumbersome components that must remain stationary and that are not easily disassembled and/or transported should it become necessary to manufacture the hot-mix asphalt at another location. An example of such large, cumbersome components is the typical conventional rotary dryer and its associated pollution control equipment.
Asphalt plants with rotary dryers are well-known in the art. Such asphalt plants, however, tend to suffer from at least one of several disadvantages. Among the disadvantages are that: 1) they are excessively, complex, large or bulky and cannot be transported easily to a location where the hot-mix asphalt is needed, 2) they use excessively expensive, complicated, and/or large pollution control devices (some of which require frequent maintenance and monitoring), and/or 3) they are not compatible with recycled asphalt products (RAP) or cannot accept commingled RAP and virgin asphalt aggregates.
Conventional rotary dryers typically use cyclones, knock-out boxes, large bag houses, and/or other space-consuming pollution control devices to process the emissions from the rotary dryers. Still other rotary dryers use expensive and/or complicated thermal oxidizers, indirect heating systems where the RAP aggregate is not exposed to the heated air, or a second rotary dryer into which the polluted exhaust gases are inserted and diluted or burned. Notably, thermal oxidizer-based systems tend to be very expensive, bulky, and have large operating costs because of their high energy requirements. Indirect heating systems tend to be thermodynamically inefficient, expensive to build, and very limited in production rates. Systems that depend on a second rotary dryer disadvantageously prevent one rotary dryer from being used without another.
The space-consuming pollution control devices typically are not easy to disassemble and transport. Often transportation of such devices requires a significant amount of disassembly and/or unusual transportation equipment and techniques. The combined weight and/or size of a typical rotary dryer and its associated pollution control devices usually exceeds the size and/or weight restrictions of wide-load trucking in the United States. It therefore is difficult, if not impossible, to transport such systems using fewer than three truckloads, and without any complicated and time-consuming disassembly of the rotary dryer and its associated pollution control equipment. There is consequently a need in the art for a hot-mix asphalt manufacturing system, such as a rotary dryer and its associated pollution control equipment, that can be transported in fewer than three truck loads, without complicated disassembly of the manufacturing system. Since wide-load trucking requires official permits, escorts, and can be performed only within certain regulatory limits, the use of wide-load trucking tends to be far more expensive, time-consuming, and less practical than conventional trucking. Conventional trucking (i.e., trucking without escorts and without wide-load designations) can be performed, according to U.S. regulations, when the load is no larger than 8 feet, 6 inches wide by 13 feet, 6 inches high by 53 feet long. The need for a transportable hot-mix asphalt manufacturing system therefore further extends to one that, without complicated disassembly of the manufacturing system, does not require loads that exceed the dimensional limits of conventional trucking, and preferably one that can be transported in fewer than three such conventional truck-loads.
If a rotary dryer is capable of handling recycled asphalt product (RAP), it typically will be configured with an external combustion chamber to protect the contents of the dryer""s drum from the combustion process that is used to generate heat. The cumulative length of the rotary dryer therefore typically includes the length of the dryer""s rotatable drum plus the length of the external combustion chamber. The additional length contributed by the combustion chamber usually precludes the rotary dryer from being transported using conventional trucking in a single truck-load. The aforementioned need to provide a readily transportable hot-mix manufacturing system therefore extends to one that has a rotary dryer with a combustion chamber does not contribute significantly, if at all, to the length of the rotary dryer.
While there are some pollution control devices that are more compact, less expensive, and/or less maintenance intensive than the pollution control devices typically found on a conventional rotary dryer, such devices generally have not found their way into the rotary dryer industry. Presumably, this is because of perceived incompatibilities with the emissions from the typical rotary dryer. Plate collectors, for example, though they are fairly compact and inexpensive, are not used as pollution control equipment in the typical rotary dryer. It is generally perceived that plate collectors would be overburdened and/or clogged by the particulates in the exhaust gas of the typical rotary dryer. This is especially so if the plate collector is to be located in the recirculated exhaust gas stream of a rotary dryer. It is generally perceived that excessive maintenance and/or replacement of the plate collectors would be necessary if such collectors were used to remove particulate from the recirculated exhaust gas stream.
While plate collectors can be cleaned by continuously spraying them with water, such xe2x80x9cwetxe2x80x9d processing generally is not used in the context of rotary dryers because the emissions from such rotary dryers typically are treated in bag houses or using other fabric-based filters. Such bag-houses and other fabric-based filters typically are incompatible with condensed water. When such fabric-based filters are used, it is typically necessary that any moisture in the filtered emissions remain in the vapor state.
Likewise, fiber bed filters are not generally used to treat the pollution from the typical hot-mix rotary dryer. Presumably, this is, in part, because of the temperature limitations imposed by the use of such fiber bed filters. The typical fiber bed filter is not compatible with hot emissions that exceed a temperature of about 120 degrees F.
It is not unusual for the emission temperature from a rotary dryer to exceed 200 degrees F. Since the emissions from the typical hot-mix rotary dryer far exceed the 120 degree temperature limitation, the general perception in the industry of hot-mix manufacturing is that fiber bed filters are not suitable for use as pollution control equipment in a hot-mix rotary dryer.
While some fiber bed filters have been provided with evaporative cooling systems, whereby water is sprayed through the emissions and evaporates to draw heat away from the emissions, the use of such fiber bed filters in the asphalt industry generally has been limited to treatment of relatively low-moisture asphalt emissions (e.g. 5 emissions from shingle manufacturing and asphalt storage facilities) having a much lower moisture content than the emissions from the typical hot-mix rotary dryer. While evaporative cooling can be effective with low-moisture emissions, such evaporative cooling techniques alone generally are not effective in the context of the moisture-saturated emissions from the typical rotary dryer. In particular, the typical emissions from the rotary dryer can accept little, if any, additional moisture. Fiber bed filters therefore, even if augmented to include evaporative cooling systems, generally have not been used to treat the emissions from a hot-mix rotary dryer.
Another problem with conventional hot-mix asphalt manufacturing systems and methods, relates to the restrictions imposed on their feed material. Many conventional hot-mix manufacturing cold feed systems, for example, are not compatible with recycled asphalt products (RAP). In the few rotary dryers and associated techniques that can handle significant amounts of RAP, the RAP and virgin aggregates typically must be fed into the rotatable drum of the rotary dryer through different inlets of the drum. The RAP and virgin aggregates therefore cannot be commingled in the typical system or method. Because it is generally more convenient to feed the RAP and virgin aggregates into a rotary dryer from the same location and/or inlet, there is a need in the art for a hot-mix manufacturing technique and/or system that includes a rotary dryer capable of receiving the raw materials in the form of commingled RAP and virgin aggregates and/or in the form of 100% RAP.
It is a primary object of the present invention to overcome at least one of the foregoing problems and/or satisfy at least one of the foregoing needs by providing a transportable hot-mix asphalt manufacturing system and method, a method and system for treating emissions from a hot-mix asphalt manufacturing system, and a method and system for treating recirculated gases in a hot-mix asphalt manufacturing system.
To achieve these and other objects, the present invention provides a transportable hot-mix asphalt manufacturing system comprising a rotary dryer and at least one pollution control device. The rotary dryer is adapted to receive and dry ingredients of hot-mix asphalt. The pollution control device(s) is (are) adapted to treat emissions from the rotary dryer so that hydrocarbons and particulates are substantially removed from the emissions of the rotary dryer before such emissions are released into a surrounding environment of the rotary dryer. The pollution control device(s) and the rotary dryer have dimensions that permit use of conventional trucking to transport the rotary dryer and the pollution control device(s) in less than three conventional truck-loads.
Also provided by the present invention is a counter-flow rotary dryer for a hot-mix asphalt manufacturing system. The counter-flow rotary dryer comprises a rotatable drum, an inlet, a combustion chamber, and a heat shield. The rotatable drum has a first end and an opposite end. The inlet is for raw ingredients of hot-mix asphalt. The inlet is located at or near the first end of the rotary dryer. The combustion chamber is internal to the rotatable drum of the rotary dryer and is radially spaced apart from an inside circumferential wall of the rotatable drum, to provide a mixing zone between the combustion chamber and the inside circumferential wall. The heat shield is adapted to protect the raw ingredients of hot-mix asphalt from radiant heat developed in the combustion chamber. The counter-flow rotary dryer is adapted to receive the raw ingredients of hot-mix asphalt through the inlet for passage through the rotatable drum toward the opposite end thereof, while combustion gases from the combustion chamber flow substantially from the opposite end of the rotary dryer toward the first end to heat and dry the raw ingredients.
Also provided by the present invention is a rotary dryer for a hot-mix asphalt manufacturing system, wherein the rotary dryer comprises a rotatable drum, a combustion chamber, a recirculator, and a plate collector. The rotatable drum has a first end and an opposite end. The combustion chamber is located at or near the opposite end. The recirculator is adapted to recirculate some combustion gases at the first end back tdthe combustion chamber at or near the opposite end. The plate collector is adapted to substantially remove particulates from combustion gases that are recirculated by the recirculator.
According to another aspect of the present invention, a hot-mix asphalt manufacturing system can be provided with a rotary dryer and at least one pollution control device that includes a fiber bed filter. The rotary dryer is adapted to receive and dry ingredients of hot-mix asphalt. The pollution control device(s) is (are) adapted to treat emissions from the rotary dryer so that hydrocarbons and particulates are substantially removed from the emissions of the rotary dryer before such emissions are released into a surrounding environment of the rotary dryer.
Also provided by the present invention is a transportable hot-mix asphalt manufacturing system comprising a counter-flow rotary dryer, a plate collector, a fiber bed filter, a cooling zone, a heat exchanger, a coolant recovery mechanism, and a plate collector sprayer. The counter-flow rotary dryer is adapted to receive and dry ingredients of hot-mix asphalt. The counter-flow rotary dryer includes a combustion chamber and a recirculator adapted to recirculate emissions from the rotary dryer into the combustion chamber. The combustion chamber is internal to the rotary dryer and is radially spaced apart from an inside circumferential wall of the rotary dryer, to provide a mixing zone between the combustion chamber and the inside circumferential wall. The plate collector is adapted to remove particulates from emissions that are recirculated into the combustion chamber by the recirculator. The fiber bed filter is adapted to treat emissions from the rotary dryer so that hydrocarbons and particulates are substantially removed from the emissions of the rotary dryer before such emissions are released into a surrounding environment of the rotary dryer. The cooling zone is located between the rotary dryer and the fiber bed filter. The cooling zone is adapted to cool the emissions from the rotary dryer enough that such emissions achieve a temperature that is compatible with the fiber bed filter. The cooling zone includes at least one coolant sprayer adapted to spray a coolant through the emissions as such emissions pass through the cooling zone. The heat exchanger is connected at least indirectly to the coolant sprayer. The heat exchanger is adapted to remove heat from the coolant before the coolant is sprayed through the cooling zone. The coolant recovery mechanism is connected at least indirectly to the heat exchanger. The coolant recovery mechanism is adapted to recover and recirculate through the heat exchanger the coolant after it has been sprayed through the cooling zone. The plate collector sprayer is adapted to spray the plate collector so that at least some of the particulates removed by the plate collector from the emissions are cleaned away from the plate collector. The hot-mix asphalt manufacturing system has dimensions that permit use of conventional trucking to transport the hot-mix asphalt manufacturing system in less than three conventional truck-loads.
The present invention also provides a method of manufacturing hot-mix asphalt. The method comprises the steps of providing a rotary dryer and at least one pollution control device, with dimensions that permit use of conventional trucking to transport the rotary dryer and the pollution control device(s) in less than three conventional truck-loads, feeding primary ingredients of hot-mix asphalt into the rotary dryer, drying the primary ingredients of hot-mix asphalt in the rotary dryer, and treating emissions from the rotary dryer so that hydrocarbons and particulates are substantially removed from the emissions of the rotary dryer before such emissions are released into a surrounding environment of the rotary dryer.
The step of providing the rotary dryer and the pollution control device(s) preferably includes configuring the pollution control device(s) and/or the rotary dryer so that its length is less than or equal to about 53 feet, its height is less than or equal to about 13 feet, 6 inches, and its width is less than or equal to about 8 feet, 6 inches.
Also provided by the present invention is a method of manufacturing hot-mix asphalt, wherein the method comprises the step of providing a rotary dryer that includes 1) a rotatable drum having a first end and an opposite end, 2) an inlet for primary ingredients of hot-mix asphalt, the inlet being located at or near the first end of the rotary dryer, 3) a combustion chamber that is internal to the rotatable drum of the rotary dryer and that is radially spaced apart from an inside circumferential wall of the rotatable drum, to provide a mixing zone between the combustion chamber and the inside circumferential wall, and 4) a heat shield adapted to protect the primary ingredients of hot-mix asphalt from radiant heat developed in the combustion chamber. This method further comprises the steps of 1) introducing primary ingredients of hot-mix asphalt into the rotatable drum, through the inlet, 2) rotating the rotatable drum so that the primary ingredients are conveyed through the rotatable drum toward the opposite end thereof, while combustion gases from the combustion chamber flow substantially from the opposite end of the rotary dryer toward the first end to heat and dry the primary ingredients, and 3) introducing supplemental ingredients into the mixing zone of the rotary dryer so that the supplemental ingredients are mixed with the primary ingredients after the primary ingredients have substantially completed a drying treatment in the rotary dryer.
Preferably, the supplemental ingredients are selected from the group of supplemental ingredients consisting of asphalt cement, rejuvinators, plasticizers, and combinations thereof.
The present invention also provides a method of treating recirculated gases in a hot-mix asphalt manufacturing system. The method comprises the steps of directing recirculated gases from a rotary dryer through a plate collector, and removing particulates from the recirculated gases as the particulates are propelled, by the recirculated gases, into the plate collector.
Also provided is a method of treating emissions from a hot-mix asphalt manufacturing system. The method comprises the steps of directing emissions from a rotary dryer to a fiber bed filter, and substantially removing hydrocarbons and particulates from those emissions at the fiber bed filter before such emissions are released into a surrounding environment of the rotary dryer.
The present invention also provides a method of manufacturing hot-mix asphalt wherein the method comprises the steps of feeding primary ingredients of hot-mix asphalt into a rotary dryer, drying the primary ingredients of hot-mix asphalt in the rotary dryer, in a counter-flow manner, treating emissions from the rotary dryer by passing the emissions through a fiber bed filter so that hydrocarbons and particulates are substantially removed from the emissions of the rotary dryer before such emissions are released into a surrounding environment of the rotary dryer, recirculating emissions from the rotary dryer back into a combustion chamber of the rotary dryer, and removing particulates, using a plate collector., from the emissions that are recirculated back into the combustion chamber.
Preferably, the emissions being processed according to any of the foregoing methods or systems are subjected to coalescent filtration and/or Brownian diffusion filtration in a fiber bed filter. In addition, the primary ingredients that are processed in any one of the foregoing systems or according to any one of the foregoing methods can include 100% recycled asphalt product (RAP) or commingled RAP with virgin aggregates. The above and other objects and advantages will become more readily apparent when reference is made to the following description taken in conjunction with the accompanying drawings.